Optical system

ABSTRACT

A zoom optical system provides a series of lens units designed to allow for zooming with minimal change in aberration resulting from the movement of the lens units. Each lens unit has a plurality of lens units. One embodiment has, in order from the object side, a first lens unit having a negative refractive power, a second lens unit having a positive refractive power, a third lens unit having a positive refractive power and a fourth lens unit having a positive refractive power wherein the second lens unit may have at least one gradient index lens element and the first lens unit is kept stationary during a change of magnification. Additional embodiments have more or fewer lens units and the refractive power of each lens unit is varied. In some embodiments, a lens unit other than the first lens unit is kept stationary.

This is a division of application Ser. No. 09/088,376, filed Jun. 2,1998, now U.S. Pat. No. 6,163,410.

BACKGROUND OF THE INVENTION

a) Field of the Invention

The present invention relates to an optical lens system which is to beused in various kinds of optical devices.

b) Description of the Prior Art

It is generally desirable for a zoom optical system to correctaberrations independently in each lens unit so that aberrations are notonly corrected in a standard position but also varied little by changinga magnification. To meet this goal, each lens unit is usually composedof a plurality of lens elements.

Recently, compact configuration and lowering of manufacturing cost aredesirable for zoom optical devices to be used in various kinds ofoptical devices. For making a zoom optical system more compact, it issufficient from the paraxial theory to increase the refractive power ofa lens unit which has a vari-focal function, thereby shortening thedistance which the lens unit must move to produce a given change ofmagnification. When a refractive power of the lens unit is increased,however, the lens unit produces greater to reduce the amount ofaberration it is necessary, to compose the lens unit of an increasednumber of lens elements. When the lens unit is composed of an increasednumber of lens elements, it is impossible to maintain compact size andlow cost. This problem sets limits to compact configuration andreduction in manufacturing costs of zoom optical systems which arecomposed of homogeneous lens elements.

Zoom optical systems may be made more compact by using radial typegradient index lens elements which have refractive index distributionsin media thereof in radial directions from optical axes.

Since a radial type gradient index lens element has a refractive indexdistribution, it may better correct aberrations than may a homogeneouslens element. Owing to a refractive index of the medium in particular,the radial type gradient index lens element has a characteristicexcellent in correction of a Petzval's sum and chromatic aberration.

As conventional examples of optical systems which use radial typegradient index lens elements, there are known lens systems disclosed,for example, as fifth and sixth embodiments of Japanese Patent KokaiPublication No. Sho 61-231517, a second embodiment of Japanese PatentKokai Publication No. Sho 61-248015, and third embodiment of JapanesePatent Kokai Publication No. Hei 2-79013. However, each of these zoomoptical systems has a zoom ratio on the order of 3 and is composed of alarger number of lens elements, or 9 to 13 lens elements.

SUMMARY OF THE INVENTION

A primary object of the present invention is to provide an opticalsystem which is composed of a small number of lens elements and can bemanufactured at a low cost.

The optical system according to the present invention in one embodimentis characterized in that it is composed, in order from the object side,of a first lens unit having a negative refractive power, a second lensunit having a positive refractive power, a third lens unit having anegative refractive power and a fourth lens unit having a positiverefractive power, and that the second lens unit comprises at least onegradient index lens element.

This optical system according to the present invention is a zoom opticalsystem whose magnification is changed by moving the second lens unitalong an optical axis.

A second embodiment of the present invention is characterized in that itis composed, in order from the object side, of a first lens unit havinga negative refractive power, a second lens unit having a positiverefractive power and a third lens unit having a positive refractivepower, that a magnification of the optical system is changed mainly bymoving the second lens unit along an optical axis and that the opticalsystem comprises at least one gradient index lens element.

Further, a third embodiment is characterized in that it is composed of aplurality of lens units, that it comprises an aperture stop and at leastone gradient index lens element, that a lens unit which comprises thegradient index lens element is moved along an optical axis mainly forchanging a magnification of the optical system and that the aperturestop is moved along the optical axis together with the lens unit whichis moved for the change of the magnification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 12 show sectional views illustrating compositions offirst through twelfth embodiments of the optical system according to thepresent invention;

FIGS. 13A, 13B and 13C show diagrams illustrating conditions of rayswhen stops are disposed before and after a gradient index lens element;and

FIGS. 14A, 14B and 14C show diagrams illustrating electronic cameras inwhich the optical system according to the present invention isassembled.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The optical system according to the present invention in one embodimentis characterized in that it is composed, in order from the object side,of a first lens unit which has a negative refractive power, a secondlens unit which has a positive refractive power, a third lens unit whichhas a negative refractive power and a fourth lens unit which has apositive refractive power, and that the second lens unit comprises atleast one gradient index lens element which has a refractive indexdistribution. Further, the optical system according to the presentinvention changes its magnification by moving the second lens unit alongan optical axis.

A radial type gradient index lens element has provides greatercorrection of aberrations than a homogeneous lens element. Forcorrecting aberrations independently in each lens unit which is to beused for composing a zoom optical system, the lens unit is generallycomposed of a plurality of lens elements. When the lens unit is composedof a small number of lens elements to lower a manufacturing cost of thezoom optical system, the lens unit produces aberrations in largeamounts, thereby making it difficult to obtain favorable imagingperformance. Use of a radial type gradient index lens element makes itpossible to compose a compact lens system of fewer lens elements thanwith homogeneous lens elements, and at a low cost.

For manufacturing a compact zoom optical system at a low cost, it isdesirable to dispose at least one radial type gradient index lenselement in a lens unit which contributes to a change of a magnification.

When a refractive power of the lens unit which contributes to the changeof the magnification is increased to configure a zoom optical systemcompact, it is difficult to correct chromatic aberration and a Petzval'ssum which are produced in amounts largely dependent on refractive power.These aberrations cannot be corrected even by using an asphericalsurface and must be corrected by using an increased number of lenselements when the zoom optical system is composed only of homogeneouslens elements. For correcting chromatic aberration and the Petzval'ssum, it is desirable to configure the lens unit so as to comprise atleast one radial type gradient index lens element which is excellent incorrection of these aberrations. By using a radial type gradient indexlens element in the lens unit which contributes to the change of themagnification in particular, it is possible to favorably correct thePetzval's sum and chromatic aberration without increasing the number oflens elements even when the refractive power of the lens unit isincreased for producing a compact lens system. In a zoom optical systemwhich is composed of four lens units, or first negative, secondpositive, third negative and fourth positive lens units, for example, itis desirable to use a radial type gradient index lens element in thesecond lens unit which has a main vari-focal function.

For this reason, the optical system adopts the composition which isdescribed above.

Further, the optical system according to the present invention which hasa second embodiment characterized in that it is composed, in order fromthe object side, of a first lens unit which has a negative refractivepower, a second lens unit which has a positive refractive power and athird lens unit which has a positive refractive power, that amagnification of the optical system is changed mainly by moving thesecond lens unit along an optical axis and that the optical system usesat least one radial type gradient index lens element.

This zoom optical system according to the present invention can also bemade compact and manufactured at a low cost.

For the optical system according to the present invention which is azoom optical system composed of the three units, one negative and twopositive lens units, it is also desirable to use a radial type gradientindex lens element in the second lens unit which mainly contributes tothe change of the magnification.

In case of a homogeneous lens element, amounts of a Petzval's sum andchromatic aberration to be produced are determined once a refractivepower and a kind of glass material are determined. In case of a radialtype gradient index lens element, in contrast, it is possible to setamounts of these aberrations to be produced at desired values bycontrolling each parameter as described above.

It is possible to set amounts of aberrations at desired values by usinga radial type gradient index lens element in a zoom optical system andcontrolling each parameter.

A refractive index distribution in a radial type gradient index lenselement can be approximated by the following square formula (a):$\begin{matrix}{{n_{\lambda}(r)} = {\sum\limits_{i = 0}^{\infty}{N_{i\quad \lambda}r^{2i}}}} & (a)\end{matrix}$

wherein the reference symbol r represents a radial distance from theoptical axis, the reference symbol N_(λ)(r) designates a refractiveindex at a radial distance of r for a wavelength of λ and the referencesymbol N_(iλ) denotes a refractive index distribution coefficient of the2i′th order for the wavelength λ.

Further, longitudinal chromatic aberration of the first order, i.e.,chromatic aberration PAC for the d-line, C-line and F-line, and aPetzval's sum PTZ are expressed by formulae (b) and (c) respectively:

PAC=K(φ_(S) /V _(0d)+φ_(m) /V _(1d))  (b)

PTZ=φ _(S) /N _(0d)+φ_(m) /N _(0d) ²  (c)

wherein the reference symbol K represents a constant which is dependenton a height of an axial ray and an angle of the axial ray on a finalsurface, the reference symbol V_(0d) designates an Abbe's number of theradial type gradient index lens element on the optical axis for thed-line, the reference symbol V_(id) denotes a value expressingdispersion corresponding to the refractive index distributioncoefficient of the 2i′th order, the reference symbol φ_(s) represents arefractive power of surface of the radial type gradient index lens forthe d-line when the lens element is considered as a thin lens elementand the reference symbol φ_(m) designates a refractive power of mediumof the radial type gradient index lens element:

V_(0d), V_(id) and φ_(m) being given by formulae (d), (e) and (f) shownbelow respectively:

V _(0d)=(N _(0d)−1)/(N _(0F) −N _(0C))  (d)

V _(id) =N _(id)/(N _(iF) −N _(iC))(1=1, 2, 3, . . . )  (e)

φ_(m)≈−2N _(1d) t _(G)  (f)

wherein the reference symbol t_(G) represents a thickness of the radialtype gradient index lens element.

As apparent from the formulae (b) and (c), chromatic aberration and aPetzval's sum of a radial type gradient index lens element which isconsidered as an independent lens element can be controlled as desiredby selecting adequate values for the second terms which is dependent onthe refractive power in the formulae. When the refractive power has anextremely small value, for example, the second terms are nearly zeroed,thereby making it difficult to correct chromatic aberration and aPetzval's sum mentioned above.

For the optical system according to the present invention which has thecomposition described above, it is desirable that the radial typegradient index lens element satisfies the following condition (1):

0.01<|N _(1d) ·t _(G)|<1  (1)

When the condition (1) is satisfied, it is possible to favorably correctchromatic aberration and a Petzval's sum. If the lower limit of 0.01 ofthe condition (1) is exceeded, the refractive power will be weakened,thereby making it difficult to correct chromatic aberration and aPetzval's sum favorably. If the upper limit of 1 of the condition (1) isexceeded, in contrast, chromatic aberration and a Petzval's sum will beovercorrected.

Further, it is desirable for correcting chromatic aberration and aPetzval's sum more favorably, it is desirable to satisfy, in place ofthe condition (1), the following condition (1-1):

0.02<|N _(1d) ·t _(G)|<0.2  (1-1)

As apparent from the formula (f), it is sufficient for increasing arefractive power of a radial type gradient index lens element to enlargea value of the distribution coefficient of the second order N_(1d) or adifference Δn between refractive index distributions of the optical axisand a marginal portion or thicken the lens element. However, Δn can beenlarged only within a certain limit in practical manufacturing ofradial type gradient index lens materials. When a gradient indexmaterial is prepared by an ion exchange method or a sol-gel method, forexample, a long time is required for imparting a refractive indexdistribution which has a large value of Δn, thereby posing problems ofhigh manufacturing cost. Accordingly, it is impossible to extremelyenlarge Δn of a radial type gradient index lens element.

In order to correct chromatic aberration and a Petzval's sum effectivelyin the optical system according to the present invention, it isdesirable that the thickness t_(G) satisfies the following condition(2):

0.05<t _(G) /{square root over (f_(W)·f_(T)+L )}<2  (2)

wherein the reference symbols f_(W) and f_(T) represent focal lengths ofthe optical system as a whole at a wide position and a tele positionrespectively.

When the condition (2) is satisfied, it is possible to impart arefractive power to a medium which is sufficient for correctingchromatic aberration and a Petzval's sum without extremely enlarging Δn.If the lower limit of 0.05 of the condition (2)is exceeded, chromaticaberration and a Petzval's sum will be undercorrected. If the upperlimit of 2 is exceeded, in contrast, these aberrations will undesirablybe overcorrected.

It is more desirable to satisfy, in place of the condition (2), thefollowing condition (2-1):

0.1<t _(G) /{square root over (f_(W)·f_(T)+L )}<1.5  (2-1)

Further, it is still more desirable to satisfy the following condition(2-2):

0.3<t _(G) /{square root over (f_(W)·f_(T)+L )}<1  (2-2)

The optical system according to the present invention has as a thirdembodiment an optical system, characterized in that it is composed of aplurality of lens units and comprises an aperture stop, that at leastone lens element disposed in the optical system is a gradient index lenselement having a refractive index distribution in its medium, that thelens unit which has a gradient index lens element is moved along anoptical axis to change a magnification and that the aperture stop ismoved along the optical axis together with the moving lens unit.

When a gradient index lens element is used in a lens unit which is movedto change a magnification in a zoom optical system, it is desirable tomove the gradient index lens element along an optical axis together withan aperture stop. By adopting such a composition, heights of off-axialrays incident on the gradient index lens element are kept nearlyconstant independently of zoom conditions, thereby making it possible tocorrect aberrations favorably in all the conditions.

If the gradient index lens element and the aperture stop are not movedalong the optical axis together with each other it will be difficult tocorrect offaxial aberrations such as coma in particular in all theconditions from the wide position to the tele position.

The ability to move the gradient index lens element and the aperturestop along the optical axis together with each other provides a meritfrom a viewpoint of manufacturing cost in addition to the merit forcorrection of aberrations. This is because off-axial rays are keptrelatively low in the vicinities of the aperture stop and makes itpossible to reduce the diameter of the gradient index lens element. Whenthe gradient index lens element has a small diameter, the manufacturingcost of the gradient index lens element is low, thereby making itpossible to manufacture the optical system at a low cost. For lowering amanufacturing cost of the optical system, it is effective to use agradient index lens element even when the gradient index lens element isimmovable.

When a gradient index lens element is to be disposed in the vicinity ofan aperture stop, this lens element is not limited to a radial typegradient index lens element but may be an axial type gradient index lenselement or an aspherical lens element. The gradient index lens elementand the aperture stop can be assembled in the same lens barrel member sothat they are moved along an optical axis. Further, the gradient indexlens element and the aperture stop may be assembled in separate lensbarrel members and moved along the optical axis respectively.

Furthermore, it is desirable that a refractive power of a radial typegradient index lens element has a sign (positive or negative sign) whichis the same as that of a refractive power of the lens unit which is touse the radial type gradient index lens element. Such a sign of therefractive power provides an advantage to correct a Petzval's sum inparticular.

As apparent from the formula (c), the denominator of the second term ofthe formula expressing a Petzval's sum of a radial type gradient indexis squared. Therefore, it is possible to configure a radial typegradient index lens element so as to produce a Petzval's sum which issmaller than that to be produced by a homogeneous lens element having arefractive power which is the same as that of the radial type gradientindex lens element. Accordingly, it is desirable to impart a refractivepower to a radial type gradient index lens element having a sign whichis the same as that of a refractive power of the lens unit. When aradial type gradient index lens element is to be used in a lens unitwhich mainly contributes to a change of a magnification in particular,such a sign of a refractive power is desirable to configure compactly anoptical system. When a radial type gradient index lens element is to beused in a second lens unit having a main vari-focal function in anoptical system which has the composition described above, or iscomposed, in order from the object side, of a first lens unit having anegative refractive power, a second lens unit having a positiverefractive power and a subsequent lens unit(s), for example, it isdesirable that a medium of the radial type gradient index lens elementhas a positive refractive power.

When a second lens unit has a main vari-focal function in a zoom opticalsystem which is composed of four lens units in the order: negative,positive, negative and positive or three lens units in the order:negative, positive and positive, it is preferable to select a lens unitother than the second lens unit as a lens unit movable for correcting adeviation of an image surface caused by changing a magnification.

In case of an optical system in which two or more lens units havevari-focal functions, it is possible to configure the optical systemcompactly and lower the manufacturing cost of the optical system byusing the radial type gradient index lens element in either of the lensunits which have the vari-focal functions. In a case where a second lensunit and a third lens unit have vari-focal functions in a vari-focaloptical system which is composed of a first lens unit having a positiverefractive power, a second lens unit having a negative refractive power,a third lens unit having a positive refractive power and subsequent lensunit or units, for example, it is possible to configure the opticalsystem compactly and lower the manufacturing cost of the optical systemby using a radial type gradient index lens element in either the secondlens unit or the third lens unit or in both.

Since a planar surface can be polished at a lower cost than a sphericalsurface, it is desirable that a radial type gradient index lens elementbe used in the zoom optical system according to the present inventionwhich has any one of the compositions described above has at least oneplanar surface. A radial type gradient index lens element which has twoplanar surfaces is more desirable to lower a manufacturing cost of theoptical system.

It is desirable to configure an optical system which uses an imagepickup device such as a CCD so that off-axial rays are incident at smallangles onto an image surface for preventing light intensities from beinginsufficient at marginal portions of an image. In order to reduce anglesof incidence of off-axial rays on an image surface, it is desirable todispose a positive lens element or a positive cemented lens component onthe image side in the optical system.

In order to compose a compact optical system of a small number of lenselements and at a low cost, it is desirable to dispose a positive lenselement or a positive lens component on the image side in the lenssystem composed of a plurality of lens units and arrange a radial typegradient index lens element on the object side of the positive lenselement or the positive lens component. By selecting the compositiondescribed above, it is possible to obtain a zoom optical systemconsisting of a small number of lens elements since the compositionallows a light bundle coming from the radial type gradient index lenselement to be focused onto an image surface while a lens unit comprisingthe radial type gradient index lens element is moved along an opticalaxis.

In order to configure an optical system which requires a certain lengthof back focal length so as to be compact at a low cost, it is desirableto configure it as a lens system composed of a plurality of lens unitsincluding a lens unit wherein a positive lens element is disposed on theimage side, a negative lens element is disposed on the object side ofthis positive lens element and a radial type gradient index lens elementis disposed on the object side of this negative lens element. Thenegative lens element disposed in this lens unit has a main function toincrease the back focal length of the lens system.

In the zoom optical system according to the present invention which hasthe third composition consisting of the plurality of lens units, it isdesirable that a first lens unit disposed on the object side is keptstationary during a change of a magnification. When the first lens unitis kept stationary, it is possible to configure a lens barrel so as tohave enhanced strength to external impact and pressure. If the firstlens unit is movable, a movable mechanism of a lens barrel may bedamaged by an external impact or pressure.

For correcting aberrations in the zoom optical system which consists ofthe plurality of lens units, it is desirable that lens unit which isdisposed on the image side is kept stationary during the change of themagnification. Since the lens unit which is disposed on the image sidemainly has an imaging function, it is possible to keep variations ofaberrations within narrow ranges by keeping this lens unit stationary.

In order to correct chromatic aberration sufficiently with a radialgradient index lens element in the zoom optical system which has any oneof the first, second and third compositions, it is desirable to satisfythe following condition (3):

1/V _(1d)<0.15  (3)

If the condition (3) is not satisfied, chromatic aberration will beundercorrected.

It is more preferable to satisfy, in place of the condition (3), thefollowing condition (3-1):

−0.1<1/V _(1d)<0.1  (3-1)

If the upper limit of 0.1 of the condition (3-1) is exceeded, chromaticaberration will be undercorrected. If the lower limit of −0.1 isexceeded, chromatic aberration will be overcorrected.

It is more desirable to satisfy, in place of the condition (3-1), thefollowing condition (3-2):

−0.02<1/V _(1d)<0.02  (3-2)

When the zoom optical system is to be used as an optical system whichmust form an image having a precise quality in particular, it isdesirable to satisfy the following condition (3-3):

−0.01<1/V _(1d)<0.01  (3-3)

The optical system according to the present invention described above isused as an optical system for silver salt cameras, video cameras,digital cameras, endoscopes, image pickup systems, measuring instrumentsand so on.

For manufacturing a radial type gradient index lens material, forexample, by the ion exchange method or the sol-gel method, it is noteasy to control a refractive index distribution with high precision anda refractive index distribution may deviate from a design value at anouter circumferential portion of the lens material. When the deviationis corrected by working an outside diameter, the lens material which hasa refractive index distribution deviated from the design value can beused as a satisfactory lens material. It is possible to obtain arelatively satisfactory lens material, for example, by cutting off theouter circumferential portion of the lens material or adding atransparent material such as a resin. Further, it is possible to disposea stop so as to shield the outer circumferential portion so as toprevent rays from passing through this portion.

Since radial type gradient index lens elements allow chromaticaberration to be controlled to a desired value as described above, it ispossible to compose, using radial type gradient index lens elements, aprism optical system which produces only a small chromatic aberration.

When an optical axis is refracted by imparting a prism function to aglass material, chromatic aberration is usually produced due to arefractive index of a refracting surface which is different fordifferent wavelengths. When a prism optical system is composed of radialtype gradient index lens elements, it is possible to correct chromaticaberration favorably in the prism optical system as a whole bycontrolling chromatic aberration to be produced by a medium to a desiredvalue. In this case, it is desirable to use a single radial typegradient index lens element for manufacturing the optical system at alow cost.

As various kinds of optical systems are configured more compactly, lenselements used in these optical systems tend to have smaller diameters.When lens elements have smaller diameters, however, it is difficult toassemble a plurality of lens elements with high precision. Inparticular, it is not easy to assemble lens elements having diameters of10 mm or smaller with high precision. Accordingly, it is desired tocompose optical systems of small numbers of lens elements and it ispreferable also for this purpose to use gradient index lens elements.Use of a gradient index lens element makes it possible to obtain anoptical system which has favorable optical performance and is composedof lens elements in a number smaller than that of homogeneous lenselements, thereby making lens assembly easier. Further, since gradientindex lens elements are likely to be thick and have thick edges, theselens element can easily be held and assembled with high precision.

It is effective to use gradient index lens elements as lens elementswhich have diameters of 10 mm or smaller or more effective to use themas lens elements which have diameters of 5 mm or smaller. Whenproduction of gradient index lens elements is taken into consideration,however, it is desirable that gradient index lens elements havediameters of at least 0.1 mm, and gradient index lens elements whichhave extremely small diameters can hardly be held and worked. For thisreason, it is more desirable gradient index lens elements have diametersof at least 0.2 mm. It is therefore desirable that gradient index lenselements to be used in the optical system according to the presentinvention has a diameter not smaller than 0.1 mm and not larger than 10mm. It is more desirable that the gradient index lens element to be usedin the optical system according to the present invention has a diameterwhich is not smaller than 0.2 mm and not larger than 5 mm.

Further, a radial type gradient index lens element which has a smalldiameter can be manufactured at a low cost and is desirable from aviewpoint of a manufacturing cost of the optical system. It is thereforedesirable for the optical system according to the present invention touse an image pickup device which has an image plane having a diagonallength not exceeding ½ inch. It is more desirable to use an image pickupdevice which has an image plane having a diagonal length not exceeding ⅓inch.

In the recent years where image pickup devices such as CCDs have pictureelements at higher densities, image pickup devices tend to have smallerdiameters. It is therefore desirable to adopt gradient index lenselements also for optical systems which use small image pickup devices.It is effective in particular to adopt gradient index lens elements foroptical systems which use image pickup devices such as CCDs having imageplanes not exceeding ½ inch in diagonal lengths (image height of 4 mm).It is more effective to adopt gradient index lens elements in opticalsystems which use image pickup devices such as CCDs having image planesnot exceeding ⅓ inch in diagonal lengths (image height of 3 mm).

Further, a refractive index distribution of a gradient index material tobe used for the optical system according to the present invention isapproximated by the square equation expressed as the formula (a).However, a refractive index material which has a refractive indexdistribution expressed by a formula other than the formula (a) can alsobe approximated by the formula (a) and used for the optical systemaccording to the present invention.

Furthermore, the optical system according to the present invention isnot limited those embodiments and examples discussed herein but may havea composition which substantially satisfies the requirements defined bythe claims.

Optical systems preferred as the embodiments of the present inventionhave compositions illustrated in FIGS. 1 through 11 and numerical datawhich is listed below:

Embodiment 1 f = 6.56˜10.78˜18.91, F nunber = 3.6˜4.7˜6.2 2ω =60.4°˜36°˜20° r₁ = ∞ d₁ = 0.8000 n₁ = 1.81600 ν₁ = 46.62 r₂ = 5.6195 d₂= 0.4788 r₃ = 6.1295 d₃ = 0.9983 n₂ = 1.84666 ν₂ = 23.78 r₄ = 11.5281 d₄= D₁ (variable) r₅ = ∞ (stop) d₅ = 0.5000 r₆ = ∞ d₆ = 5.9864 n₃(gradient index lens element) r₇ = ∞ d₇ = D₂ (variable) r₈ = 11.6948 d₈= 0.8000 n₄ = 1.74077 ν₄ = 27.79 r₉ = 5.7435 d₉ = D₃ (variable) r₁₀ =15.2627 d₁₀ = 1.4009 n₅ = 1.88300 ν₅ = 40.76 r₁₁ = −26.2228 d₁₁ = 0.1993r₁₂ = ∞ d₁₂ = 1.6000 n₆ = 1.51633 ν₆ = 64.14 r₁₃ = ∞ d₁₃ = 1.6000 n₇ =1.51633 ν₇ = 64.14 r₁₄ = ∞ d₁₄ = 0.4000 r₁₅ = ∞ d₁₅ = 0.7500 n₈ =1.51633 ν₈ = 64.14 r₁₆ = ∞ d₁₆ = 1.1255 r₁₇ = ∞ (image) f 6.56 10.7818.91 D₁ 11.06493 5.95704 0.50000 D₂ 5.81906 6.47619 11.96499 D₃ 1.821666.33050 6.36121 gradient index lens element wavelength N₀ N₁ d line1.65000 −9.2600 × 10⁻³ C line 1.64512 −9.2557 × 10⁻³ F line 1.66138−9.2700 × 10⁻³ Embodiment 2 f = 6.68˜10.93˜19.39, F number = 2.4˜3.1˜4.22w = 60°˜36°˜19.8° r₁ = −361.5471 d₁ = 1.0000 n₁ = 1.81600 ν₁ = 46.62 r₂= 7.0279 d₂ = 0.8270 r₃ = 7.6415 d₃ = 2.4141 n₂ = 1.84666 ν₂ = 23.78 r₄= 12.9691 d₄ = D₁ (variable) r₅ = ∞ (stop) d₅ = 0.5000 r₆ = ∞ d₆ =4.7683 n₃ (gradient index lens element) r₇ = ∞ d₇ = D₂ (variable) r₈ =11.9747 d₈ = 0.9999 n₄ = 1.84666 ν₄ = 23.78 r₉ = 6.5959 d₉ = D₃(variable) r₁₀ = 21.3761 d₁₀ = 1.8432 n₅ = 1.88300 ν₅ = 40.76 r₁₁ =−21.3051 d₁₁ = 1.0000 r₁₂ = ∞ d₁₂ = 1.6000 n₆ = 1.51633 ν₆ = 64.14 r₁₃ =∞ d₁₃ = 1.6000 n₇ = 1.51633 ν₇ = 64.14 r₁₄ = ∞ d₁₄ = 1.0000 r₁₅ = ∞ d₁₅= 0.7500 n₈ = 1.51633 ν₈ = 64.14 r₁₆ = ∞ d₁₆ = 1.1681 r₁₇ = ∞ (image) f6.68 10.93 19.39 D₁ 14.87539 8.67516 2.00000 D₂ 7.18816 8.25510 15.35672D₃ 0.95504 6.09207 5.68683 gradient index lens element wavelength N₀ N₁N₂ d line 1.75000 −9.4769 × 10⁻³ 4.7643 × 10⁻⁶ C line 1.74250 −9.4684 ×10⁻³ 4.7643 × 10⁻⁶ F line 1.76750 −9.4968 × 10⁻³ 4.7643 × 10⁻⁶Embodiment 3 f = 6.58˜10.05˜18.18, F number = 3.8˜4.0˜4.8 2w =59.8°˜36.2°˜20° r₁ = 16.8604 d₁ = 2.1134 n₁ = 1.83481 ν₁ = 42.72 r₂ =−391.7314 d₂ = D₁ (variable) r₃ = 103.5216 d₃ = 1.0000 n₂ = 1.61800 ν₂ =63.33 r₄ = 6.9508 d₄ = 1.4921 r₅ = −7.4105 d₅ = 1.0000 n₃ = 1.72916 ν₃ =54.68 r₆ = −68.1757 d₆ = D₂ (variable) r₇ = ∞ (stop) d₇ = 1.0000 r₈ =87.3294 d₈ = 5.8926 n₄ (gradient index lens element) r₉ = ∞ d₉ = 4.0561r₁₀ = 12.5839 d₁₀ = 1.0000 n₅ = 1.84666 ν₅ = 23.78 r₁₁ = 8.5354 d₁₁ =5.1373 r₁₂ = 13.2633 d₁₂ = 1.2214 n₆ = 1.83481 ν₆ = 42.72 r₁₃ = 136.3787d₁₃ = D₃ (variable) r₁₄ = ∞ d₁₄ = 1.8000 n₇ = 1.61700 ν₇ = 62.80 r₁₅ = ∞d₁₅ = 0.2000 r₁₆ = ∞ d₁₆ = 0.7500 n₈ = 1.51633 ν₈ = 64.14 r₁₇ = ∞ d17 =1.1715 r₁₈ = ∞ (image) f 6.58 10.05 18.18 D₁ 1.06245 2.85460 3.49323 D₂10.72360 6.43554 0.50000 D₃ 0.49996 2.33703 8.32438 gradient index lenselement wavelength N₀ N₁ d line 1.65000 −9.2600 × 10⁻³ C line 1.64512−9.2557 × 10⁻³ F line 1.66138 −9.2700 × 10⁻³ Embodiment 4 f =6.12˜10.37˜17.41, F number = 3.1˜3.3˜3.4 2ω = 64°˜37.8°˜22.8° r₁ =21.5966 d₁ = 2.4811 n₁ = 1.84666 ν₁ = 23.78 r₂ = 83.9279 d₂ = D₁(variable) r₃ = 2148.0415 d₃ = 0.7999 n₂ = 1.77250 ν₂ = 49.60 r₄ =7.7737 d₄ = D₂ (variable) r₅ = ∞ (stop) d₅ = 1.0000 r₆ = ∞ d₆ = 7.0573n₃ (gradient index lens element) r₇ = ∞ d₇ = D₃ (variable) r₈ = 25.3819d₈ = 0.7999 n₄ = 1.84666 ν₄ = 23.78 r₉ = 6.9348 d₉ = D₄ (variable) r₁₀ =160.7405 d₁₀ = 2.0020 n₅ = 1.84666 ν₅ = 23.78 r₁₁ = −9.8318 d₁₁ = D₅(variable) r₁₂ = ∞ d₁₂ = 1.6000 n₆ = 1.51633 ν₆ = 64.14 r₁₃ = ∞ d₁₃ =1.6000 n₇ = 1.51633 ν₇ = 64.14 r₁₄ = ∞ d₁₄ = 1.5000 r₁₅ = ∞ d₁₅ = 0.7500n₈ = 1.51633 ν₈ = 64.14 r₁₆ = ∞ d₁₆ = 1.1568 r₁₇ = ∞ (image) f 6.1210.37 17.41 D₁ 1.89674 6.02579 9.83419 D₂ 15.19674 7.32933 1.00000 D₃2.27608 0.14807 0.42202 D₄ 0.92932 3.69229 5.15328 D₅ 0.19982 3.321834.12121 gradient index lens element wavelength N₀ N₁ N₂ d line 1.65000−9.2600 × 10⁻³ 1.0785 × 10⁻⁵ C line 1.64512 −9.2557 × 10⁻³ 1.0780 × 10⁻⁵F iine 1.66138 −9.2700 × 10⁻³ 1.0796 × 10⁻⁵ g line 1.67083 −9.2518 ×10⁻³ 1.1185 × 10⁻⁵ Embodiment 5 f = 4.75˜8.67˜13.94, F number =2.7˜3.8˜4.8 2ω = 71°˜40.5°˜24.2° r₁ = 295.4949 d₁ = 1.6000 n₁ = 1.84666ν₁ = 23.78 r₂ = −33.3209 d₂ = 0.2000 r₃ = −61.1954 d₃ = 0.8000 n₂ =1.61800 ν₂ = 46.62 r₄ = 5.4615 d₄ = 2.0212 r₅ = 6.3586 d₅ = 1.6000 n₃ =1.84666 ν₃ = 23.78 r₆ = 7.8635 d₆ = D₁ (variable) r₇ = ∞ d₇ = 6.9390 n₄(gradient index lens element) r₈ = ∞ d₈ = D₂ (variable) r₉ = 6.7708 d₉ =0.7997 n₅ = 1.84666 ν₅ = 23.78 r₁₀ = 4.5573 d₁₀ = D₃ (variable) r₁₁ =26.7807 d₁₁ = 1.8000 n₆ = 1.81600 ν₆ = 46.62 r₁₂ = −11.9170 d₁₂ = 0.1999r₁₃ = ∞ d₁₃ = 1.6000 n₇ = 1.51633 ν₇ = 64.14 r₁₄ = ∞ d₁₄ = 1.6000 n₈ =1.51633 ν₈ = 64.14 r₁₅ = ∞ d₁₅ = 0.4000 r₁₆ = ∞ d₁₆ = 0.7500 n₉ =1.51633 ν₉ = 64.14 r₁₇ = ∞ d₁₇ = 1.1419 r₁₈ = ∞ (image) f 4.75 8.6713.94 D₁ 11.45422 5.39034 1.00000 D₂ 3.98909 5.04270 11.13732 D₃ 2.204907.18586 5.47117 gradient index lens element wavelength N₀ N₁ N₂ d line1.80000 −8.3994 × 10⁻³ 6.9866 × 10⁻⁶ C line 1.79200 −8.3821 × 10⁻³6.9866 × 10⁻⁶ F line 1.81867 −8.4397 × 10⁻³ 6.9866 × 10⁻⁶ g line 1.83483−8.4356 × 10⁻³ 6.9866 × 10⁻⁶ e line 1.80630 −8.4145 × 10⁻³ 6.9866 × 10⁻⁶Embodiment 6 f = 4.22˜7.46˜11.59, F number = 3.5˜3.7˜3.9 2ω =59.2°˜32.6°˜21° r₁ = −83.2538 d₁ = 0.9563 n₁ = 1.72916 ν₁ = 54.68 r₂ =13.7980 d₂ = D₁ (variable) r₃ = ∞ (stop) d₃ = 1.0000 r₄ = 8.2784 d₄ =6.1835 n₂ (gradient index lens element) r₅ = 3.7725 d₅ = 3.4709 r₆ =9.3241 d₆ = 1.5000 n₃ = 1.72916 ν₃ = 54.68 r₇ = −21.8979 d₇ = D₂(variable) r₈ = ∞ d₈ = 1.8000 n₄ = 1.61700 ν₄ = 62.80 r₉ = ∞ d₉ = 0.2000r₁₀ = ∞ d₁₀ = 0.7500 n₅ = 1.51633 ν₅ = 64.14 r₁₁ = ∞ d₁₁ = 1.2424 r₁₂ =∞ (image) f 4.22 7.46 11.5 D₁ 23.99294 7.93421 0.50000 D₂ 1.000002.93580 5.39558 gradient index lens element wavelength N₀ N₁ d line1.65000 −9.2600 × 10⁻³ C line 1.64512 −9.2557 × 10⁻³ F line 1.66138−9.2700 × 10⁻³ Embodiment 7 f = 7.82˜10.73˜23.55, F number = 2.6˜3.2˜5.02ω = 45.8°˜23°˜15° r₁ = −87.9283 d₁ = 1.0110 n₁ = 1.88300 ν₁ = 40.76 r₂= 8.5547 d₂ = 0.3932 r₃ = 8.7336 d₃ = 3.9684 n₂ = 1.68893 ν₂ = 31.08 r₄= 94.0338 d₄ = D₁ (variable) r₅ = ∞ (stop) d₅ = 1.0502 r₆ = ∞ d₆ =11.8703 n₃ (gradient index lens element) r₇ = 5.0684 d₇ = D₂ (variable)r₈ = 21.3858 d₈ = 2.4007 n₄ = 1.60300 ν₄ = 65.44 r₉ = −19.6447 d₉ = D₃(variable) r₁₀ = ∞ d₁₀ = 2.0200 n₅ = 1.51633 ν₅ = 64.14 r₁₁ = ∞ d₁₁ =1.6000 n₆ = 1.51633 ν₈ = 64.14 r₁₂ = ∞ d₁₂ = 1.6000 r₁₃ = ∞ d₁₃ = 0.7500n₇ = 1.51633 ν₇ = 64.14 r₁₄ = ∞ d₁₄ = 1.1586 r₁₅ = ∞ (image) f 7.8210.78 23.55 D₁ 20.16326 15.52315 2.07349 D₂ 2.42907 8.51751 21.98237 D₃1.61275 0.19996 0.19997 gradient index lens element wavelength N₀ N₁ N₂d line 1.58000 −6.0000 × 10⁻³ 2.0040 × 10⁻⁵ C line 1.57652 −5.9775 ×10⁻³ 1.9965 × 10⁻⁵ F iine 1.58812 −6.0525 × 10⁻³ 2.0216 × 10⁻⁵Embodiment 8 f = 6.55˜11˜19.46, F number = 3.6˜4.8˜6.4 2ω =60.4°˜35.6°˜14.1° r₁ = 40.9080 d₁ = 0.7999 n₁ = 1.69680 ν₁ = 55.53 r₂ =5.7662 d₂ = 1.4033 r₃ = 6.1102 d₃ = 1.2293 n₂ = 1.84666 ν₃ = 23.78 r₄ =7.3191 d₄ = D₁ (variable) r₅ = ∞ (stop) d₅ = 0.5000 r₆ = 10.7147 d₆ =1.2317 n₃ = 1.77250 ν₃ = 49.60 r₇ = −202.6535 d₇ = 2.7200 r₈ = 15.4248d₈ = 1.6245 n₄ = 1.88300 ν₄ = 40.76 r₉ = −7.0963 d₉ = 0.1988 r₁₀ =−6.0819 d₁₀ = 0.8000 n₅ = 1.84666 ν₅ = 23.78 r₁₁ = 59.2972 d₁₁ = D₂(variable) r₁₂ = 15.5725 d₁₂ = 0.8000 n₆ = 1.54814 ν₆ = 45.78 r₁₃ =5.7769 d₁₃ = D₃ (variable) r₁₄ = 13.2623 d₁₄ = 1.9556 n₇ = 1.88300 ν₇ =40.76 r₁₅ = −40.0039 d₁₅ = 0.1998 r₁₆ = ∞ d₁₆ = 1.6000 n₈ = 1.51633 ν₈ =64.14 r₁₇ = ∞ d₁₇ = 1.6000 n₉ = 1.51633 ν₉ = 64.14 r₁₈ = ∞ d₁₈ = 0.4000r₁₉ = ∞ d₁₉ = 0.7500 n₁₀ = 1.51633 ν₁₀ = 64.14 r20 = ∞ d₂₀ = 1.1663 r₂₁= ∞ (image) f 6.55 11 19.46 D₁ 11.53674 5.96880 0.50000 D₂ 2.095592.70333 7.71887 D₃ 2.60453 7.57840 8.04678 Embodiment 9 f = 7.52, Fnumber = 2.9, 2ω = 59° r₁ = ∞ (stop) d₁ = 0.5000 r₂ = −4.2544 d₂ =3.3444 n₁ (gradient index lens element) r₃ = −8.1332 d₃ = 3.0948 r₄ =10.1395 d₄ = 3.9600 n₂ = 1.80610 ν₂ = 40.92 r₅ = −4.6038 d₅ = 0.7983 n₃= 1.84666 ν₃ = 23.78 r₆ = −25.3258 d₆ = 2.4766 r₇ = ∞ d₇ = 1.6000 n₄ =1.51633 ν₄ = 64.14 r₈ = ∞ d₈ = 1.6000 n₅ = 1.51633 ν₅ = 64.14 r₉ = ∞ d₉= 0.5000 r₁₀ = ∞ d₁₀ = 0.7500 n₆ = 1.48749 ν₆ = 70.23 r₁₁ = ∞ d₁₁ =1.1912 r₁₂ = ∞ (image) gradient index lens element wavelength N₀ N₁ dline 1.65000 −9.2600 × 10⁻³ C line 1.64487 −9.2558 × 10⁻³ F line 1.66197−9.2699 × 10⁻³ Embodiment 10 r₁ = ∞ (stop) d₁ = 15.0000 n₁ (gradientindex lens element) r₂ = ∞ d₂ = 0.0925 r₃ = ∞ (image) gradient indexlens element wavelength N₀ N₁ d line 1.67800 −9.0245 × 10⁻³ C line1.67265 −9.0959 × 10⁻³ F iine 1.69049 −8.8577 × 10⁻³ Embodiment 11 f =4.89˜8.63˜20.4, F number = 3.2˜4.1˜6.6 2ω = 76.6°˜44.4°˜19.2° r₁ =17.8104 d₁ = 2.6456 n₁ = 1.74100 ν₁ = 52.64 r₂ = 6.7693 d₂ = 2.5884 r₃ =330.8548 d₃ = 0.8000 n₂ = 1.60300 ν₂ = 65.44 r₄ = 8.6751 d₄ = 0.7126 r₅= 8.4625 d₅ = 1.6000 n₃ = 1.84666 ν₃ = 23.78 r₆ = 16.3699 d₆ = D₁(variable) r₇ = ∞ (stop) d₇ = 0.5000 r₈ = ∞ d₈ = 5.4448 n₄ (gradientindex lens element) r₉ = −40.2775 d₉ = D₂ (variable) r₁₀ = 7.1860 d₁₀ =2.8854 n₅ = 1.78472 ν₅ = 25.68 r₁₁ = 4.5503 d₁₁ = D₃ (variable) r₁₂ =29.4393 d₁₂ = 2.0000 n₆ = 1.69680 ν₆ = 55.53 r₁₃ = −12.3722 d₁₃ = 0.2000r₁₄ = ∞ d₁₄ = 1.6000 n₇ = 1.51633 ν₇ = 64.14 r₁₅ = ∞ d₁₅ = 1.6000 n₈ =1.51633 ν₈ = 64.14 r₁₆ = ∞ d₁₆ = 0.4000 r₁₇ = ∞ d₁₇ = 0.7500 n₉ =1.51633 ν₉ = 64.14 r₁₈ = ∞ d₁₈ = 1.1493 r₁₉ = ∞ (image) f 4.89 8.63 20.4D₁ 17.65743 8.86991 1.00000 D₂ 4.75469 6.19762 16.16667 D₃ 2.000005.65005 9.32004 gradient index lens element wavelength N₀ N₁ N₂ d line1.70000 −6.2418 × 10⁻³ −1.1880 × 10⁻⁵ C line 1.69475 −6.2588 × 10⁻³−1.1912 × 10⁻⁵ F line 1.71225 −6.2021 × 10⁻³ −1.1804 × 10⁻⁵ g line1.72241 −6.1441 × 10⁻³ −1.1589 × 10⁻⁵

wherein the reference symbols r₁, r₂, . . . represent radii of curvatureon surfaces of respective lens elements, the reference symbols d1, d2, .. . designate thicknesses of the respective lens elements and airspacesreserved therebetween, the reference symbols n₁, n₂, . . . denoterefractive indices of the respective lens elements, and the referencesymbols ν₁/ν₂, . . . represent Abbe's numbers of the respective lenselements.

The first embodiment is a zoom optical system which has a compositionillustrated in FIG. 1, or is composed, in order from the object side, ofa first lens unit having a negative refractive power, a second lens unithaving a positive refractive power, a third lens unit having a negativerefractive power and a fourth lens unit having a positive refractivepower. Plane parallel plates disposed after the fourth lens unitrepresent filters such as a low pass filter and an infrared filter, anda cover glass plate of an image pickup device.

In the first embodiment, the first lens unit is kept stationary duringchange of a magnification, and has a function to lead an axial ray andoff-axial rays to the second lens unit, the second lens unit is movablefor changing the magnification and has a main vari-focal function, thethird lens unit is movable for changing the magnification and has a mainfunction to correct a deviation caused by changing the magnification,and the fourth lens unit is kept stationary during the change of themagnification and has a function to image a light bundle coming from thethird lens unit.

As understood from the foregoing description, the first embodiment is azoom optical system which consists of the four negative, positive,negative and positive lens units, and can be in a compact configurationat a low cost by using a radial type gradient index lens element. Thoughthe first embodiment is compact, it is capable of correcting aberrationsowing to a radial type gradient index lens element used in the secondlens unit having the vari-focal function for strengthening a refractivepower of this lens unit. Use of the radial type gradient index lenselement makes it possible to compose the second lens unit of a singlelens element and manufacture the optical system at a low cost.

Further, the optical system according to the present invention can befocused on an object located at an extremely short distance by movingthe third lens unit which is a negative lens element along an opticalaxis. The optical system can be focused also by moving the first lensunit along the optical axis.

For favorably correcting mainly off-axial aberrations such as lateralchromatic aberration and distortion, the first lens unit is composed, inorder from the object side, of a negative lens element and a positivelens element. Further, it is possible by keeping the first lens unitstationary during the change of the magnification to configure a lensbarrel so as to enhance its strength to external impact and pressure.

Furthermore, it is possible to suppress aberrations caused by changingthe magnification by keeping the lens unit disposed on the image side(the fourth lens unit) stationary during the change of themagnification.

In the optical system preferred as the first embodiment, a stop isdisposed on the object side of the second lens unit and moved togetherwith the second lens unit for changing the magnification, therebyreducing variations of aberrations caused by the change of themagnification. Owing to the fact that the radial type gradient indexlens element used as the second lens unit is moved together with thestop, the radial type gradient index lens element can have a smalldiameter and the optical system can be manufactured at a low cost.Further, the third lens unit and the fourth lens unit which are disposedon the image side of the radial type gradient index lens element arecomposed of a negative lens element and a positive lens elementrespectively. That is, disposed on the object side of the radial typegradient index lens element are a negative lens element and a positivelens element in order from the object side so that the optical systemcan be composed of a small number of lens elements. In other words, atriplet type lens system is composed a positive lens element, a negativelens element and a positive lens element on the image side of the firstlens unit which has a function to widen a field angle so thataberrations can be corrected effectively with a small number of lenselements. Owing to the fact that the positive lens element is disposedon the image side, it is possible to allow off-axial rays to be incidenton an image surface nearly in parallel with the optical axis.Furthermore, all the lens elements other than the gradient index lenselement are homogeneous spherical lens elements so as to reduce adverseinfluence due to eccentricity of the optical system and allow theoptical system to be manufactured at a low cost. Moreover, the radialtype gradient index lens element has two planar surfaces so that theoptical system can be manufactured at a low cost.

The second embodiment is a zoom optical system which has a compositionillustrated in FIG. 2, or is composed, in order from the object side, ofa first lens unit which is kept stationary during a change of amagnification and has a negative refractive power, a second lens unitwhich is movable for changing the magnification and has a positiverefractive power, a third lens unit which is movable for the change ofthe magnification and has a negative refractive power, and a fourth lensunit which is kept stationary during the change of the magnification andhas a positive refractive power. A radial type gradient index lenselement is used in the second lens unit. In other words, the secondembodiment is an optical system which is composed of four lens units,negative, positive, negative and positive in that order from the objectside, and can be configured compactly at a low cost by using a radialtype gradient index lens element.

Though the lens units of the second embodiment have functions which aresubstantially the same as those of the lens elements of the firstembodiment, the second embodiment has an F number which is smaller thanthat of the first embodiment and is configured as a brighter opticalsystem. Though the second lens unit therefore tends to produce sphericalaberration in a large amount, spherical aberration is correctedfavorably by using the radial type gradient index lens element in thesecond lens unit. Though the second lens unit which has the positiverefractive power tends to remarkable negative spherical aberration,spherical aberration is corrected favorably by selecting a positivevalue for the distribution coefficient N_(2λ) of the fourth order of thegradient index lens element so that it produces positive sphericalaberration.

The third embodiment is a zoom optical system which has a compositionillustrated in FIG. 3, or is composed, in order from the object side, ofa first lens unit having a positive refractive power, a second lens unithaving a negative refractive power and a third lens unit having apositive refractive power.

The first lens unit is kept stationary during a change of amagnification and has a function to lead an axial light bundle and anoff-axial light bundle to the second lens unit, whereas the second andthird lens units are movable for changing the magnification, and have avari-focal function and another function to vary an airspace reservedbetween these lens units for correcting a deviation of an image surfacewhich is caused by changing the magnification.

The third embodiment is a zoom optical system which consists of thethree positive, negative and positive lens units, and is made compact ata low cost by using a radial type gradient index lens element.

The third embodiment is a zoom optical system which comprises a positivelens unit disposed on the object side. It is made compact by using theradial type gradient index lens element in the third lens unit whichmainly has the vari-focal function, thereby strengthening the refractivepower of this lens unit and corrects aberrations favorably in spite ofthe strengthened refractive power. The first lens unit is composed of asingle positive lens element, the second lens unit is composed of twonegative lens elements and the third lens unit is composed of threepositive, negative and positive lens elements: the lens element disposedon the object side in the third lens unit being configured as the radialtype gradient index lens element. A stop is disposed on the object sideof the third lens unit and moved together with the third lens unit so asto lower a manufacturing cost by reducing a diameter of the radial typegradient index lens element. The third embodiment is configured to befocused on an object located at an extremely short distance by movingthe negative lens element in the third lens unit along an optical axis.The third embodiment can be focused also by moving the positive lenselement which is disposed on the object side in the third lens unit. Thethird embodiment is configured as an optical system which is composed ofa small number of lens elements and has favorable optical performance bydisposing, in order from the object side, the negative lens element andthe positive lens element on the image side of the radial type gradientindex lens element in the third lens unit. All the lens elements otherthan the radial type gradient index lens element are homogeneousspherical lens elements so as to reduce influences due to eccentricitiesand lower manufacturing cost. Furthermore, the radial type gradientindex lens element has two planar surfaces so that it can bemanufactured at a low cost.

The fourth embodiment is an optical system having a compositionillustrated in FIG. 4, or is a zoom optical system which is composed, inorder from the object side, of a first lens unit having a positiverefractive power, a second lens unit having a negative refractive power,a third lens unit having a positive refractive power, a fourth lens unithaving a negative refractive power and a fifth lens unit having apositive refractive power.

In this optical system, the first lens unit is kept stationary during achange of a magnification and leads an axial light bundle and anoff-axial light bundle to the second lens unit, the second and thirdlens units are movable for changing the magnification and mainly have avari-focal function, the fourth lens unit mainly corrects deviationcaused due to the change of the magnification, and the fifth lens unithas the function of imaging a light bundle coming from the fourth lensunit onto an image surface. The fourth embodiment is an example of zoomoptical system which consists of five positive, negative, positive,negative and positive lens elements, and is configured compactly at alow cost by using a radial type gradient index lens element in theoptical system.

Though the fourth embodiment is a zoom optical system which comprises apositive lens unit disposed on the object side, it is configuredcompactly by using a radial type gradient index lens element in a thirdlens unit which mainly has a vari-focal function, thereby strengtheningthe refractive power of this lens unit and correcting aberrationsfavorably in spite of the strengthened refractive power of the thirdlens unit. Further, each of the lens units is composed of a single lenselement.

Further, a stop is disposed on the object side of the third lens unitand moved together with the third lens unit so as to reduce variationsof aberrations caused by a change of a magnification. The movement ofthe stop together with the third lens unit permits reducing a diameterof the radial type gradient index lens element and lower a manufacturingcost. Furthermore, the fourth lens unit is composed of a single negativelens element and the fifth lens unit is composed of a single positivelens element disposed on the image side of the radial type gradientindex lens element or a negative lens element and a positive lenselement disposed on the image side of the radial type gradient indexlens element, thereby configuring the optical system compact.

The optical system preferred as the fourth embodiment is configured tobe focused on an object located at an extremely short distance by movingthe fourth lens unit along an optical axis. The fourth embodiment can befocused on an object located at an extremely short distance also bymoving the positive lens element of the fifth lens unit. All the lenselements other than the radial type gradient index lens element arehomogeneous spherical lens elements, whereby the optical system isscarcely influenced by eccentricities and can therefore be manufacturedat a low cost. Furthermore, the radial type gradient index lens elementhas two planar surfaces, thereby also serving for reduction ofmanufacturing cost.

The fifth embodiment is an optical system having a compositionillustrated in FIG. 5, or is a zoom optical system which is composed, inorder from the object side, of a first lens unit having a negativerefractive power, a second lens unit having a positive refractive power,a third lens unit having a negative refractive power and a fourth lensunit having a positive refractive power.

In this optical system, the first lens unit is kept stationary during achange of a magnification and leads an axial light bundle and anoff-axial light bundle to the second lens unit, the second lens unit ismovable for changing the magnification and mainly has a vari-focalfunction, the third lens unit is movable for changing the magnificationand mainly functions to correct a deviation of an image surface causedby changing the magnification, and the fourth lens unit is keptstationary during the change of the magnification and images a lightbundle coming from the third lens unit. The fifth embodiment is a zoomoptical system which is composed of four negative, positive, negativeand positive lens units, and configured compactly by using a radial typegradient index lens element in the optical system.

The fifth embodiment is configured as an optical system having a fieldangle which is made wider than that of the first embodiment by composingthe first lens unit of three lens elements. Further, the optical systemis configured compactly and aberrations are corrected favorably by usingthe radial type gradient index lens element in the second lens unitwhich has the vari-focal function so as to strengthen the refractivepower of the second lens unit. Furthermore, the use of the gradientindex lens element makes it possible to compose the second lens unit ofa single lens element.

For correcting off-axial aberrations such as chromatic aberration anddistortion, the first lens unit is composed, in order from the objectside, of a positive lens element, a negative lens element and a positivelens element. An aperture stop is imaginarily disposed nearly in themiddle of the radial type gradient index lens element which is thesecond lens unit, whereby the aperture stop is moved together with thesecond lens unit and variations of aberrations can be small during thechange of the magnification. Since the aperture stop is moved togetherwith the second lens unit, it is possible to lower the cost of theoptical system by reducing a diameter of the radial type gradient indexlens element. In FIG. 5, a reference symbol S represents the imaginaryaperture stop.

When an aperture stop is imaginarily disposed in a radial type asdescribed above, it is desirable to dispose actual stop mechanisms onboth the object side and the image side of the gradient index lenselement. By disposing stop mechanisms as described above, it is possibleto narrow a light bundle uniformly, reduce ununiformities within ascreen and enhance resolution by narrowing the light bundle.

FIGS. 13A, 13B and 13C are diagrams illustrating conditions of a lightbundle obtainable by disposing stop mechanisms on both sides of agradient index lens element. In the drawing, a reference symbol GLrepresents the gradient index lens element, a reference symbol AXdesignates an optical axis, a reference symbol L₁ denotes an off-axialprincipal ray, a reference symbol L₂ represents an upper subordinateray, a reference symbol L₃ designates a lower subordinate ray, areference symbol S₁ denotes a stop mechanism on the object side, areference symbol S₂ represents a stop mechanism on the image side and areference symbol S₃ designates the imaginary stop.

FIG. 13A shows a case wherein the stop mechanism S₁ is disposed on theobject side of the gradient index lens element. In this case, the uppersubordinate ray L₂ and the lower subordinate ray L₃ of the light bundleare asymmetrical with regard to the principal ray L₁ since the uppersubordinate ray L₂ is limited at a point P of the lens element GL.Accordingly, the lower subordinate ray L₃ is narrowed first and the raysare narrowed asymmetrically by stopping down the stop mechanism S₁.Since the rays are narrowed at degrees which are variable dependently onfield angles, variations in brightness and resolution are produced on ascreen by stopping down the stop mechanism S₁. In order to reduce thesevariations, it is desirable to adopt a configuration wherein stopmechanisms are disposed on both sides of the gradient index lens elementGL so as to form an imaginary aperture stop is formed in the lenselement as shown in FIG. 13B. In this composition, the upper subordinateray L₂ is limited by the stop mechanism S₂ and the lower subordinate rayL₃ is limited by the stop mechanism S₁. When the two stop mechanisms S₁and S₂ are stopped down, the upper subordinate ray and the lowersubordinate ray are narrowed symmetrically with regard to the principalray L₁ as if the imaginary stop S₃ were stopped down in the gradientindex lens element.

For the reason described above, it is desirable to dispose stopmechanisms on the object side and the image side of an optical systemsuch as the fifth embodiment which comprises an imaginary stop in a lenselement.

The sixth embodiment has a composition illustrated in FIG. 6, or is azoom optical system which is composed, in order from the object side, ofa first lens unit having a negative refractive power and a second lensunit having a positive refractive power. Both the first lens unit andthe second lens unit are movable for changing the magnification and tocorrect a deviation of an image surface by varying an airspace reservedbetween both the lens units. Though the sixth embodiment is a zoomoptical system which is composed of the two negative and positive lensunits, it can be configured compactly and manufactured at a low cost.Though the negative lens unit is disposed on the object side in thesixth embodiment, it is configured compactly and corrects aberrationsfavorably by using the radial type gradient index lens element in thesecond lens unit which mainly has the vari-focal function.

Further, an aperture stop is disposed on the object side of the secondlens unit and moved together with the second lens unit so as to reducevariations of aberrations during the change of the magnification. Sincethe aperture stop is moved together with the radial type gradient indexlens element, it is possible to reduce a diameter of this gradient indexlens element and lower the manufacturing cost. In other words, the sixthembodiment is configured to compose the optical system of a small numberof lens elements by disposing a positive lens element on the image sideof the gradient index lens element, or composing the second lens unit ofthe radial type gradient index lens element and a positive lens element.Further, all the lens elements other than the gradient index lenselement are configured as homogeneous spherical lens elements so thatthe optical system is scarcely influenced by eccentricities and can bemanufactured at a low cost.

The radial type gradient index lens element has a shape of a negativelens element for correcting a Petzval's sum and chromatic aberration inparticular, and its image side surface is configured as a concavesurface to prolong a back focal length.

The sixth embodiment is configured to be focused on an object located atan extremely short distance by moving the positive lens element disposedon the image side in the second lens unit.

The seventh embodiment has a composition illustrated in FIG. 7, or is azoom optical system which is composed, in order from the object side, ofthree lens units; a first lens unit having a negative refractive power,a second lens unit having a positive refractive power and a third lensunit having a positive refractive power.

In the seventh embodiment, the first lens unit is kept stationary duringa change of a magnification and leads an axial light bundle and anoff-axial light bundle to the second lens unit, the second lens unit ismovable for changing the magnification and mainly has a vari-focalfunction, and the third lens unit has a function mainly to correct adeviation of an image surface caused by changing the magnification. Theseventh embodiment is a zoom optical system which is composed of thethree negative, positive and positive lens units, and can be configuredcompactly and manufactured at a low cost by using the radial typegradient index lens element in the optical system.

The seventh embodiment is capable of correcting aberrations favorablythough it is a zoom optical system which comprises a negative lens unitdisposed on the image side and is configured compactly by using theradial type gradient index lens element in the second lens unit whichmainly has the vari-focal function so as to strengthen a refractivepower of this lens unit. Since an aperture stop is disposed on theobject side of the gradient index lens element which is the second lensunit and movable together with the second lens unit, it is possible toreduce variations of aberrations during the change of the magnification,reduce a diameter of the radial type gradient index lens element andmanufacture the optical system compactly. Further, a lens componentwhich is disposed on the image side of the gradient index lens element,i.e., the third lens unit, is composed of a single lens element, wherebythe optical system as a whole is composed of a small number of lenselements. Furthermore, all the lens elements other than the gradientindex lens element are configured as homogeneous lens elements, wherebythe optical system is scarcely influenced by eccentricities and can bemanufactured at a low cost. Moreover, one of surfaces of the radial typegradient index lens element is configured as a planar surface to lower amanufacturing cost.

The seventh embodiment is focused by moving the third lens unit along anoptical axis.

The eighth embodiment is a zoom optical system which is composed, inorder from the object side as shown in FIG. 8, of a first lens unithaving a negative refractive power, a second lens unit having a positiverefractive power, a third lens unit having a negative refractive powerand a fourth lens unit having a positive refractive power.

In the eighth embodiment, the first lens unit is kept stationary duringa change of a magnification and leads an axial light bundle and anoffaxial light bundle to the second lens unit, the second lens unit ismovable for changing the magnification and mainly has a vari-focalfunction, the third lens unit is movable for changing the magnificationand corrects a deviation caused by changing the magnification, and thefourth lens unit is kept stationary during the change of themagnification and has a function to image a light bundle coming from thethird lens unit. That is, the eighth embodiment is a zoom optical systemwhich is composed of the four negative, positive, negative and positivelens units like the first embodiment. Differently from the firstembodiment, however, all the lens elements used in the eighth embodimentare homogeneous lens elements.

For favorably correcting mainly off-axial aberrations such as lateralchromatic aberration and distortion in the optical system, the firstlens unit is composed, in order from the object side, of a negative lenselement and a positive lens element. Since the first lens unit is keptstationary during the change of the magnification, a lens barrel can beconfigured so as to have an enhanced strength to external impact andpressure. Further, variations of aberrations caused by changing themagnification can be suppressed since the fourth lens unit which isdisposed on the image side is kept stationary during the change of themagnification. Furthermore, an aperture stop which is disposed on theobject side of the second lens unit and moved together with the secondlens unit makes it possible to reduce variations of aberrations to becaused by changing the magnification.

The eighth embodiment is focused on an object located at an extremelyshort distance by moving the third lens unit along an optical axis. Theeighth embodiment can be focused also by moving the first lens unit orthe fourth lens unit along the optical axis. Further, the eighthembodiment can be focused by moving at least one lens element disposedin the second lens unit.

The ninth embodiment is an optical system which has a single focal pointand is composed, in order from the object side as shown in FIG. 9, of afirst lens unit having a positive refractive power and a second lensunit having a positive refractive power: the first lens unit beingcomposed of a single radial type positive refractive index lens element,and the second lens unit being composed of a cemented lens componentconsisting of a negative lens element and a positive lens element. Theninth embodiment has high imaging performance though it is composed of asmall number of lens elements, i.e., a radial type gradient index lenselement and lens elements of a positive cemented lens component. Anaperture stop is disposed on the object side. The aperture stop isdisposed in the vicinity of the radial type gradient index lens elementfor reducing a diameter of the radial type gradient index element,thereby lowering a manufacturing cost.

In the ninth embodiment, chromatic aberration and a Petzval's sum arefavorably corrected by configuring the radial type gradient index lenselement so as to have a shape of a negative lens element. Further, theradial type gradient index lens element has a meniscus shape having aconcave surface on the object side for favorably correcting sphericalaberration. The radial type gradient index lens element has the meniscusshape whose concave surface is located on the side of the aperture stopso that it is capable of favorably correcting off-axial aberrations suchas coma in particular.

The tenth embodiment is a prism optical system which is composed of asingle radial type gradient index lens element as shown in FIGS. 10A,10B or 10C. In FIG. 10A, a first surface r₁ and an optical axis AX₂ of amedium of the radial type gradient index lens element are eccentric froman optical axis AX₁ of the optical system. The optical system preferredas the tenth embodiment images a light bundle which is incidentobliquely at an angle of 27° relative to the optical axis AX₁ of theoptical system in the vicinity of a surface r₂ of the optical systemwith the single radial type gradient index lens element.

Though chromatic aberration is produced due to a prism function of thefirst surface r₁ in case of a lens element which is made of an ordinaryglass material and has such a shape as that shown in the drawing,chromatic aberration is corrected by the radial type gradient index lenselement in the tenth embodiment. In this case, it is desirable forcorrecting chromatic aberration to satisfy the condition (3). If thecondition (3) is not satisfied, chromatic aberration will beundercorrected.

The optical system preferred as the tenth embodiment is configured tosatisfy the condition (3) and corrects chromatic aberration as describedabove.

Though the first surface of the radial type gradient index lens elementis eccentric from the optical axis of the optical system in FIG. 10A, itis possible to compose a similar prism optical system by making only asurface or a medium eccentric.

In FIG. 10B, an optical axis AX₂ of a medium of a radial type gradientindex lens element is eccentric from an optical axis AX₁ of an opticalsystem. In this drawing, a reference symbol L₁ represents a principalray which is incident on the optical axis AX₁ of the optical system, andreference symbols L₂ and L₃ designate subordinate rays respectively. Aprism optical system can be obtained by making the optical axis AX₂ ofthe medium eccentric from the optical axis of the optical system asshown in this drawing.

The radial type gradient index lens element which is shown in FIG. 10Bis obtained by taking out only the portion of a radial type gradientindex lens element which is hatched in FIG. 10C and can compose theoptical system shown in FIG. 10B.

When an optical system uses at least one gradient index lens elementwhich has a refractive index distribution in its medium and an opticalaxis of a surface or medium of the gradient index lens element iseccentric from an optical axis of the optical system, it is desirablefor correcting chromatic aberration to satisfy the condition (3).

The eleventh embodiment is a zoom optical system which is composed, inorder from the object side as shown in FIG. 11, of a first lens unithaving a negative refractive power, a second lens unit having a positiverefractive power, a third lens unit having a negative refractive powerand a fourth lens unit having a positive refractive power. Planeparallel plates which are disposed on the image side of the opticalsystem, i.e., on the image side of the fourth lens unit, are filterssuch as a low pass filter and a infrared cut filter, and a cover glassplate for an image pickup device.

In the optical system preferred as the eleventh embodiment, the firstlens unit and the second lens unit are movable for changing amagnification and mainly have a vari-focal function, the third lens unitis movable for changing the magnification and mainly functions tocorrect a deviation of an image surface caused by changing themagnification, and the fourth lens unit is kept stationary during thechange of the magnification and functions to image a light bundle comingfrom the third lens unit.

The eleventh embodiment is a zoom optical system which is composed offour negative, positive, negative and positive lens units, andconfigured compact and manufactured at a low cost by using a radial typegradient index lens element in the optical system. The eleventhembodiment is capable of correcting aberrations favorably though it isconfigured compactly by using the radial type gradient index lenselement in the second lens unit which has a vari-focal function so as tostrengthen a refractive power of this lens unit. Further, the secondlens unit is composed only of the single radial gradient index lenselement, thereby lowering the manufacturing cost.

For favorably correcting off-axial aberrations such as lateral chromaticaberration and distortion, the first lens unit is composed, in orderfrom the object side, of a negative lens element, a negative lenselement and a positive lens element. Further, the lens unit which isdisposed on the image side, i.e., the fourth lens unit is keptstationary during the change of the magnification, thereby being capableof suppressing variations of aberrations due to the change of themagnification. Furthermore, an aperture stop is disposed on the objectside of the second lens unit and moved together with the second lensunit for reducing variations of aberrations during the change of themagnification. By moving the radial type gradient index lens elementtogether with the aperture stop, it is possible to reduce a diameter ofthe radial type gradient index lens element and lower a manufacturingcost. Disposed on the image side of the radial type gradient index lenselement (the second lens unit) are a negative lens element (the thirdlens unit) and a positive lens element (the fourth lens unit) in orderfrom the object side so as to compose a lens system of a small number oflens elements. Accordingly, the second lens unit, the third lens unitand the fourth lens unit which are disposed on the image side of thefirst lens unit compose a triplet type lens system, thereby beingcapable of correcting aberrations effectively with a small number oflens elements. A positive lens element is disposed on the image side soas to allow off-axial rays to be incident on an image surface nearly inparallel with an optical axis. All the lens elements other than theradial type gradient index lens element are homogeneous spherical lenselements which are effective to reduce influences due to eccentricitiesand lower a manufacturing cost.

The twelfth embodiment illustrated in FIGS. 12A, 12B and 12C exemplifiesmethods to correct an outer circumferential portion of a radial typegradient index lens element which is deviated from design values. InFIG. 12A, a reference symbol AX represents an optical axis, a referencesymbol GL designates a radial type gradient index lens element, areference symbol L₁₁ denotes an axial light bundle which has a lowaperture ratio in the vicinity of the optical axis, and a referencesymbol L₁₂ represents an axial light bundle which is apart from theoptical axis and has a high aperture ratio.

The gradient index lens element GL shown in FIG. 12A has an outercircumference in which a refractive index distribution is deviated froma satisfactory distribution and an axial ray passes as shown in thedrawing.

FIG. 12B exemplifies a working of an outer circumferential portion ofthe gradient index lens element for correcting spherical aberration.

It is possible to correct the outer circumferential portion by adding atransparent material such as a glass or plastic material as shown inFIG. 12C.

The optical system according to the present invention described above isusable as an objective optical system for image pickup apparatus such aselectronic cameras. For example, it is usable as a photographicobjective optical system for electronic cameras as shown in FIGS. 14A,14B and 14C. Out of these drawings, FIG. 14A is a perspective view of anelectronic camera as seen from the front, FIG. 14B is a perspective viewof the electronic camera as seen from the rear and FIG. 14C is a diagramillustrating an arrangement of the objective optical system in theelectronic camera. In these drawings, a reference numeral 11 representsa photographic optical system which has a photographic optical path 12,a reference numeral 13 designates a viewfinder optical system which hasan optical path 14 for a viewfinder, a reference numeral 15 denotes ashutter, a reference numeral 16 represents a flash lamp, a referencenumeral 17 designates a filter, a reference numeral 18 denotes aelectronic image pickup device, and a reference numeral 19 represents aliquid crystal monitor.

The photographic optical system 11 shown in FIG. 14C is the firstembodiment of the present invention which is illustrated in FIG. 1, andis composed, in order from the object side (in order from the left sideto the right side in FIG. 14C), of a first lens unit G₁ having anegative refractive power, a second lens unit G₂ having a positiverefractive power, a third lens unit having a negative refractive power,a fourth lens unit G₄ having a positive refractive power, the infraredcut filter 17 and the electronic image pickup device 18 which isdisposed on an image surface of the photographic optical system. Animage which is picked up by the electronic image pickup device 18 orrecorded by a recorder (not shown) in this optical system is displayedon the liquid crystal monitor 19.

I claim:
 1. An optical system comprising in order from the object side:a first lens unit having negative refractive power; a second lens unithaving positive refractive power; a third lens unit having negativerefractive power; and a fourth lens unit having positive refractivepower, wherein the first lens unit is kept stationary during a change ofa magnification and consists of two lens elements.
 2. An optical systemaccording to claim 1, wherein the first lens unit consists, in orderfrom the object side, of a negative lens element and a positive lenselement.
 3. An optical system according to claim 1, wherein amagnification is changed by varying an airspace between the first lensunit and the second lens unit and an airspace between the second lensunit and the third lens unit.
 4. An optical system according to claim 3,wherein the fourth lens unit is kept stationary during the change of themagnification.
 5. An optical system according to claim 1, wherein thethird lens unit is moved for focusing.
 6. An optical system according toclaim 1, wherein the third lens unit consists of a negative lenselement.
 7. An optical system comprising in order from the object side:a first lens unit having negative refractive power, a second lens unithaving positive refractive power, a third lens unit having negativerefractive power; and a fourth lens unit having positive refractivepower, wherein the first lens unit consists of two lens elements and thefourth lens unit consists of a positive lens element.
 8. An opticalsystem according to claim 7, wherein the first lens unit consists of anegative lens element and a positive lens element in order from theobject side.
 9. An optical system according to claim 7, wherein amagnification is changed by varying an airspace between the first lensunit and the second lens unit, an airspace between the second lens unitand the third lens unit and an airspace between the third lens unit andthe fourth lens unit.
 10. An optical system according to claim 9,wherein the fourth lens unit is kept stationary during the change of themagnification.
 11. An optical system according to claim 7, wherein thethird lens unit is moved for focusing.
 12. An optical system accordingto claim 7, wherein the third lens unit consists of a negative lenselement.
 13. An optical system comprising in order from the object side:a first lens unit having negative refractive power, a second lens unithaving positive refractive power, a third lens unit having negativerefractive power; and a fourth lens unit having positive refractivepower, wherein an aperture stop is disposed between the first lens unitand the second lens unit, and wherein a magnification is changed byvarying an airspace between the first lens unit and the second lensunit, an airspace between the second lens unit and the third lens unit,and an airspace between the third lens unit and the fourth lens unit.14. An optical system according to claim 13, wherein the third lens unitis moved to change a magnification and the aperture stop is movedtogether with the second lens unit.
 15. An optical system according toclaim 13, wherein the fourth lens unit is kept stationary during achange of the magnification.
 16. An optical system according to claim13, wherein the third lens unit is moved for focusing.
 17. An opticalsystem according to claim 13, wherein the third lens unit consists of anegative lens element.